1. Field of the Disclosure
The disclosure generally relates to a sensing circuit, and more particularly, to a photo-sensing pixel circuit and an image sensor.
2. Description of Related Art
A complementary metal-oxide-semiconductor (CMOS) image sensor is usually fabricated on a Si, GaAs, SiC, or SiGe substrate. For the purpose of image sensing, a CMOS image sensor usually includes a photo-sensing part, a signal conversion circuit, and an output circuit. The photo-sensing part senses a light source and transmits an obtained optical signal to a transmission circuit. The signal conversion circuit then converts the optical signal into an electric signal and transmits the electric signal to the output circuit.
In the operation mode described above, the conversion gain of the CMOS image sensor is determined by the equivalent capacitance on an internal node of the signal conversion circuit. If the photo-sensing part has a high full well capacity (FWC) and the CMOS image sensor has a large conversion gain, after electrons generated by the photo-sensing part are converted by the signal conversion circuit into the electric signal, the voltage obtained when the electric signal is input into the output circuit may be too low, and as a result, the entire CMOS image sensor cannot work. Or, the low voltage produced by the electric signal may cause the electrons generated by the photo-sensing part not to be successfully transmitted to the signal conversion circuit, and as a result, image lag may be produced.
The FWC of the photo-sensing part is usually reduced to resolve aforementioned problem. However, since the dynamic range of the CMOS image sensor is related to the FWC and read noise, under the same read noise condition, the dynamic range of the CMOS image sensor has to be sacrificed once the FWC of the photo-sensing part is reduced in order to maintain a high signal sensitivity inside the signal conversion circuit. In other words, even though a conventional CMOS image sensor with a high conversion gain offers a high sensitivity, the operation of a backend circuit will be restricted when the signal range of the CMOS image sensor is too large. In this case, it is impossible to achieve a large dynamic range and image lag may be produced.
A CMOS image sensor with a smaller conversion gain is designed as another resolution. Even though such a design allows the photo-sensing part to have a high FWC and a large dynamic range to be achieved, it offers a low signal sensitivity inside the signal conversion circuit. If the CMOS image sensor works in a poorly illuminated environment, a high conversion gain is required, which may result in extra noise in the backend circuit and accordingly a reduced signal-to-noise ratio (SNR). In other words, even though a conventional CMOS image sensor with a low conversion gain offers a high dynamic range, a large backend gain is required when it works in a poorly illuminated environment. As a result, the SNR of the circuit will be reduced.